Electronic devices having solder joints, such as CPUs (central processing units), have recently come to be required to have smaller sizes and higher performances. This has resulted in a tendency that the current density per terminal of the semiconductor elements mounted on such electronic devices becomes higher. It is said that in the future, the current density will reach about 104 to 105 A/cm2. An increase in current density causes electromigration in the solder joints. The progress of the electromigration may result in ruptures of the solder joints.
Electromigration (hereinafter often referred to as “EM”) is the following phenomenon. First, some of atoms constituting the solder joint collide with electrons which are producing the current, and the momentum is transferred from the electrons to the atoms. The atoms which have increased in momentum move toward the anode side of the solder joint along the flow of the electrons. The movement of the atoms toward the anode side of the solder joint results in the formation of vacancies in a cathode-side portion of the solder joint. Such vacancies gradually spread to form voids. The formed voids grow to finally cause a rupture of the solder joint. Because of the recent increases in current density, EM is becoming a serious problem.
Meanwhile, Sn—Cu solder alloys and Sn—Ag—Cu solder alloys have been extensively used as conventional lead-free solder alloys. These solder alloys are prone to suffer EM because Sn, which is used therein as a main component, has a large number of net charges. There are cases where solder joints formed from these alloys rupture when a current is caused to flow therethrough at a high current density over a long period. Although there are several other factors in solder joint ruptures, various alloys are being investigated in order to inhibit solder joint ruptures.
Patent Document 1 discloses an Sn—Bi—Cu—Ni solder alloy, which contains Bi in an amount of 2 mass % or more so that the solder alloy has improved tensile strength and improved wetting properties to give solder joints inhibited from rupturing.
Patent Document 2 discloses an Sn—Cu—Ni—Bi—Ge solder alloy, which is formed by adding Ge to the solder alloy shown above in order to improve the wetting properties to enable the solder alloy to retain high bonding strength, without decreasing in bonding strength, even when aged after solder bonding and to have improved reliability.
Patent Document 3 discloses a solder alloy based on the same type of alloy as in Patent Document 2. In Patent Document 3, the content of Bi has been reduced to less than 1% in order to inhibit the solder alloy from decreasing in impact resistance. The solder alloy of Patent Document 3 contains both P and Ge to have improved wetting properties, thereby giving solder joints having excellent impact resistance.